Electric ARC melting furnace

Abstract: An electric arc melting furnace comprises a furnace shell (1) with an electrode (3) for melting scrap by heat of arc, an air blasting port (20) for blasting air, oxygen or oxygen-enriched air into the furnace shell (1) through an air control valve (19), a carbon blasted quantity indicator (CI) for measuring quantity of carbon to be charged into the furnace shell (1), a scrap charged quantity indicator (SI) for measuring quantity of the scrap charged into the furnace shell (1), an oxygen blasted quantity indicator (OI) for measuring quantity of oxygen blasted into the furnace shell (1) and an arithmetic processor (7) for computing quantity of air required for complete combustion of carbon monoxide (23) in the furnace shell (1) in response to measured value from the indicators (CI)(SI)(OI) to control a degree of opening of the air control valve (19). Air with quantity in proportion to the carbon monoxide (23) is blasted through the air blasting port (20), which enables control of combustion temperature in the furnace shell (1) and control of temperature in the scrap preheating device.

Description: TECHNICAL FIELD The present invention relates to an electric arc melting furnace for melting scrap to reproduce refined steel. BACKGROUND ART In a conventional electric arc melting furnace as shown in Fig. 1 in vertical section, a furnace shell a has a water-cooled roof b at a center of which an electrode c is vertically provided. The furnace shell a has at its bottom an oxygen blasting port d through which oxygen is blasted into the the shell a. The furnace shell a has a spout e at its side. A spout door f outside of the spout e is vertically movable by a door cylinder g such that the spout e is closed and opened by downward and upward moment of the door f, respectively. The roof b is provided with an exhaust gas duct h which accommodates a conveyer j to charge scrap i into the shell a and which has an air suction port k. The scrap i is transported together with carboniferous material such as coke by the conveyer j, charged into the shell a and is heated and melted by heat of arc from the electrode c into molten steel l. The roof b serves to absorb radiant heat from a surface of the molten steel l. The oxygen blasted through the port d reacts with the carbon to generate carbon monoxide m which is partially burned with air n intruding into the shell a through the spout e and other gaps. Generally, the quantity of the intruding air n is not sufficient to completely burn all of the carbon monoxide m and therefore about 30-60% of the carbon monoxide m remains unburned and enters into the exhaust duct h in which the carbon monoxide m is re-burned with air from the suction port k. Resultant reaction products to be discharged from the duct h as exhaust gases o preheat the scrap i and carboniferous material transported by the conveyer j into the shell a. In the conventional electric arc melting furnace shown in Fig. 1 where the carbon monoxide m generated in the shell a is burned with the intruding air n, the quantity of the intruding air n is less as described above and the carbon monoxide m generated is partially burned so that temperature in the shell a is locally increased to generate local heat, disadvantageously resulting in failure of uniform heating of the molten steel l. The intruding air n which is relied upon for combustion in the shell a is not controlled at all in relation with the quantity of carbon monoxide m generated in the shell a and the combustion in the shell is left to take its natural course. Therefore, such combustion of the carbon monoxide m cannot be employed for control of temperature in the shell a and, in fact, hardly contributes to any enhancement of thermal efficiency in the shell a depending upon conditions. Flames are nonuniformly distributed in the shell a due to the partial combustion of the carbon monoxide by the intruding air n so that radiant heat from the surface of the molten steel l cannot be effectively absorbed by the water-cooled roof b, disadvantageously resulting in decrease of heat efficiency of the electric arc melting furnace. The carbon monoxide m, which remains unburned in the shell a in the nonuniform combustion by the intruding air n, is introduced into the duct h with its concentration being left uneven. The introduced carbon monoxide with uneven concentration is non-uniformly or locally burned in the duct h, which may cause the scrap I in the duct h to be non-uniformly preheated or may lead to explosion in the duct h. Preheating temperature of the scrap I cannot be controlled at all. In view of the above, the present invention has for its object to overcome the above and other defects encountered in the prior art and provide an electric arc melting furnace in which not only combustion temperature in a furnace shell but also preheating temperature in a scrap preheating device are controllable as desired, thereby positively ensuring a high degree of thermal efficiency and safety in furnace operation. According to the present invention an electric arc melting furnace of the type comprising a shell, an electrode for melting scrap by heat produced by the electric arc and a scrap preheating device disposed above the furnace shell for preheating the scrap to be fed to the furnace shell by means of the exhaust gases leaving the furnace shell, characterised in that an induction fan for inducing the exhaust gases from the upper outlet of the scrap preheating device through exhaust piping and discharging the induced exhaust gases, a recirculation fan for blasting a proportion of the exhaust gases at the outlet side of the induction fan through exhaust supply piping, an exhaust gas blasting port, which is connected to the exhaust supply piping and is provided on the upper portion of the furnace shell and communicates with the upper space within the furnace shell, a flow control damper in the exhaust supply piping on the outlet side of the recirculation fan, a thermometer for detecting the temperature of the upper space in the said furnace shell and an upper space temperature controller for controlling the degree of opening of the flow control damper so as to maintain the temperature detected by the thermometer at a preset value. According to a further aspect of the present invention, an electric arc melting furnace of the type referred to above is characterised in that an induction fan for inducing the exhaust gases from an upper outlet of the scrap preheating device through exhaust piping and discharging the induced exhaust gases, a recirculation fan for blasting a proportion of the exhaust gas at the outlet side of the induction fan through exhaust supply piping, an exhaust gas blasting port, which is connected to the exhaust supply piping and is provided on the lower inlet of the scrap preheating device and communicates with the interior thereof, a flow control damper in the exhaust supply piping on the outlet side of the recirculation fan, a thermometer for detecting the temperature of the lower inlet of the scrap preheating device, an inlet temperature controller for controlling the degree of opening of the flow control damper so as to maintain the temperature detected by the thermometer at a preset value, bypassing piping connecting the lower inlet of the scrap preheating device to the inlet side of the induction fan, a bypass damper in the bypass piping, a further thermometer for detecting the temperature of the upper outlet of the scrap preheating device and an outlet temperature controller for controlling the degree of opening of the bypass damper so as to maintain the temperature detected by the further thermometer at a preset vale. Further features and details of the invention will be apparent from the following description of two specific embodiments which is given by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a vertical sectional view of a conventional electric arc melting furnace, whose construction was described above; Fig. 2 is a vertical sectional view of one embodiment in accordance with the invention; and Fig. 3 is a vertical sectional view of a second embodiment in accordance with the invention. The furnace shown in Fig. 2 includes a furnace shell 1 with a peripheral wall 1a abd a water-cooled roof 2, extending vertically through the centre of which is an electrode 3. Provided at the bottom of the furnace shell 1 is an oxygen blasting port 12 for blasting oxygen into the shell. The roof 2 also has a carbon blasting port 4 and a scrap preheating device, both provided at positions remote from the centre of the roof 2. The carbon blasting port 4 charges carbonaceous material, such as coke, graphite or the like, into the shell 1. In use, iron scrap passes through the preheating device 5 into the shell 1 where it is heated and melted by the arc from the electrode 3 to form molten steel 22. Air is blasted through an air blasting port 20 in the upper space C of the shell. Carbon in the blasted carbonaceous material and in the scrap reacts with the blasted oxygen to generate heat and carbon monoxide 23. The carbon monoxide burns to form flames 24. Exhaust gases 25 flow out in the scrap preheater 5. A discharged gas blasting port 26 extends through an upper portion of the peripheral wall 1a of the furnace shell 1 and communicates with the interior of the shell 1. The scrap preheating device 5 has an upper outlet 27 which is connected through exhaust piping 28 and a gas cooler 29 to an induction fan 30. The induction fan 30 is connected at its outlet side by exhaust gas supply piping 31 to the exhaust gas blasting port 26. A recirculation fan 32 is incorporated in the piping 31 to supply part of the exhaust gases 25 from the outlet side of the induction fan 30 into the upper space C in the furnace shell 1. A flow control damper 33 is disposed at outlet side of the fan 32. A thermometer 34 is provided to detect the temperature in the upper space C of the furnace shell 1. The temperature 35 detected by the thermometer 34 and a preset temperature 36 are inputted into an upper space temperature controller 37 which is arranged to control the degree of opening of the damper 33 to maintain the temperature in the upper space C at the preset temperture 36. Reference numeral 38 denotes a flowmeter which detects the quantity of the recirculated exhaust gases in the piping 31. The exhaust gases 25 from the upper outlet 27 of the scrap preheating device 5 are cooled by the gas cooler 29 and the induced by and discharged from the induction fan 30. The exhaust gases 25 at the outlet side of the induction fan 30 are partly recirculated into the upper space C in the shell 1 through the recirculation fan 32 and the exhaust gas supply piping 31. The temperature 35 detected by the thermometer 34 is inputted into the upper space temperature controller 37 into which the preset temperature 36 is also inputted. In response to the detected value from the flowmeter 38, the degree of opening of the flow control damper 33 is controlled to adjust the flow rate of the recirculated exhaust gases 25 so that the detected temperature 35 is maintained equal to the preset temperature 36 by the controller 37. The above-mentioned recirculation of the cooled exhaust gases 25 into the upper space C makes it possible to maintain the temperature in the upper space C at the predetermined temperature 36. The discharged gases 25 fed to the upper space C interrupt the transmission of radiant heat from the molten steel 22 to the roof 2, thereby enhancing the thermal efficiency. Furthermore, the uniform dilution of the carbon monoxide 23 in the furnace shell 1 and the recirculation of the exhaust gases 25 with a low oxygen content prevent adverse problems such as oxidation, non-uniform combustion and explosion in the scrap preheating device 5. Fig. 3 illustrates a further embodiment of the invention in which parts same as those in Fig. 2 are denoted by the reference numerals. The scrap preheating device 5 has an upper outlet 27 which is connected through exhaust piping 28 and a gas cooler 29 to an induction fan 30. The induction fan 30 is connected at its outlet side by exhaust gas supply piping 41 to an exhaust gas blasting port 40 in the inlet 39 of the scrap preheating device 5. A recirculation fan 42 is incorporated in the piping 41 to supply part of the exhaust gases 25 from the outlet side of the induction fan 30 to the inlet 39. A flow control damper 43 is disposed at outlet side of the fan 42. A thermometer 44 is provided to detect the temperature in the inlet 39 of the scrap preheating device 5. An inlet temperature controller 47, into which the temperature 45 detected by the thermometer 44 and a preset temperature 46 are inputted, is arranged to control the degree of opening of the damper 43 to maintain the temperature in the lower inlet 39 at the preset temperature 46. Reference numeral 48 denotes a flowmeter which detects the quantity of the recirculated exhaust gases in the piping 41. The inlet 39 of the preheating device 5 is connected to the exhaust piping 28 at the outlet side of the upper outlet 27 through bypassing piping 49 which has a bypass damper 50. A thermometer 51 is provided to detect the temperature at the upper outlet 27 of the scrap preheating device 5. An outlet tempertature controller 54, into which the temperature 52 detected by the thermometer 51 and a preset temperature 53 are inputted, is arranged to control the degree of opening of the bypass damper 50 to maintain the temperature in the upper outlet 27 at the preset temperature 53. Reference numeral 55 notes a switch damper disposed in the bypass piping 41. The exhaust gases 25 from the upper outlet 27 of the preheating device 5 are cooled by the gas cooler 29 and then induced by and discharged from the induction fan 30. The exhaust gases 25 from the outlet side of the induction fan 30 are partly recirculated into the lower inlet 39 of the preheating device 5 through the recirculation fan 42 and the exhaust gas supply piping 41. The temperature 45 detected by the thermometer 44 is inputted into the inlet temperature controller 47 into which the preset temperature 46 is also inputted. In response to the detected value from the flowmeter 48, the degree of opening of the flow control damper 43 is controlled to adjust the flow rate of the recirculated exhaust gases 25 so that the detected temperature 45 of the thermometer 44 may be maintained equal to the preset temperature 46 by the controller 47. The above-mentioned recirculation of the cooled exhaust gases 25 to the lower inlet 39 makes it possible to maintain the temperature in the lower inlet 39 at the predetermined temperature. Furthermore, the discharged gases 25 with a low oxygen content uniformly dilute the carbon monoxide 23 in the lower inlet 39. This results in the prevention of problems such as oxidation, non-uniform combustion and explosion in the scrap preheating device 5. The exhaust gases 25 in the lower inlet 39 of the preheating device 5 are bypassed through the bypass piping 49 and the bypass damper 50 to the downstream side of the upper outlet 27. The detected temperature 52 from the thermometer 51 is inputted into the outlet temperature controller 54. The degree of opening of the bypass damper 50 is controlled to adjust the bypassed flow rate of the exhuast gases 25. The exhaust gases 25 partly bypassed through the bypass piping 49 to the downstream side of the upper outlet 27 contribute to controlling the temperature of the upper outlet 27 to a predetermined temperature, which also prevents white smoke and noxious compounds (dioxin) from being generated. Furthermore, the exhaust gases 25 may be bypassed in an excess volume for preheating of the scrap, which contributes to reducing the pressure loss in the scrap preheating device 5 and reducing the size of the induction fan 30. In the event of problems such as clogging of the scrap preheating device 5, the exhaust gases 25 may be vented to the bypass pipe 39 which can therefore also serve as a safety system.

Claims: An electric arc melting furnace comprising a shell (1), an electrode (3) for melting scrap by heat produced by the electric arc and a scrap preheating device (5) disposed above the furnace shell (1) for preheating the scrap to be fed to the furnace shell (1) by means of the exhaust gases (25) leaving the furnace shell, characterised by an induction fan (30) for inducing the exhaust gases from an upper outlet (27) of the scrap preheating device (5) through exhaust piping (28) and discharging the induced exhaust gases, a recirculation fan (32) for blasting a proportion of the exhaust gases at the outlet side of the induction fan (30) through exhaust supply piping (31), an exhaust gas blasting port (26), which is connected to the exhaust supply piping (31) and is provided on the upper portion of the furnace shell and communicates with the upper space within the furnace shell, a flow control damper (33) in the exhaust supply piping (31) on the outlet side of the recirculation fan (32), a thermometer (34) for detecting the temperature of the upper space in the said furnace shell (1) and an upper-space temperature controller (37) for controlling the degree of opening of the flow control damper (33) so as to maintain the temperature detected by the thermometer (34) at a preset value. An electric arc melting furnace comprising a shell (1), an electrode (3) for melting scrap by heat produced by the electric arc and a scrap preheating device (5) disposed above the furnace shell (1) for preheating the scrap to be fed to the furnace shell (1) by means of the exhaust gases (25) leaving the furnace shell, characterised by an induction fan (30) for inducing the exhaust gases from an upper outlet (27) of the scrap preheating device (5) through exhaust piping (28) and discharging the induced exhaust gases, a recirculation fan (42) for blasting a proportion of the exhaust gas at the outlet side of the induction fan (30) through exhaust supply piping (41), an exhaust gas blasting port (40), which is connected to the exhaust supply piping (41) and is provided on the lower inlet (39) of the scrap preheating device (5) and communicates with the interior thereof, a flow control damper (43) in the exhaust supply piping (41) on the outlet side of the recirculation fan (42), a thermometer (44) for detecting the temperature of the lower inlet (39) of the scrap preheating device (5), an inlet temperature controller (47) for controlling the degree of opening of the flow control damper (43) so as to maintain the temperature detected by the thermometer (44) at a preset value, bypass piping (49) connecting the lower inlet of the scrap preheating device (5) to the inlet side of the induction fan (30), a bypass damper (50) in the bypass piping (49), a further thermometer (51) for detecting the temperature of the upper outlet (27) of the scrap preheating device (5) and an outlet temperature controller (54) for controlling the degree of opening of the bypass damper (50) so as to maintain the temperature detected by the further thermometer (51) at a preset value.

Inventors: IURA TORU, OGUSHI MASAKI, TAKEUCHI OSAMU, YAMAMURA IKUO, YOSHIDA HIRONOBU, IURA, TORU, OGUSHI, MASAKI, TAKEUCHI, OSAMU, YAMAMURA, IKUO, YOSHIDA, HIRONOBU, Ogushi, Masaki c/o Yokohama Eng. Center, Yamamura, Ikuo, Otakebashi-Koen Sky-Heights 1103, Yoshida, Hironobu c/o Yokohama Eng. Center
Applicants: ISHIKAWAJIMA HARIMA HEAVY IND, ISHIKAWAJIMA-HARIMA JUKOGYO KABUSHIKI KAISHA
Application year: 1994
Main classification: F27B3
IPC codes: F27D19, F27D13, F27B3, C21C5